I'd like to make it with you, ma'am

Gyles Brandreth's account of the Queen's marriage, Philip and Elizabeth, is ludicrously lubricious, while Flora Fraser's account of George III daughters, Princesses, shows that the royal family is little changed

Princesses: The Six Daughters of George IIIby Flora FraserJohn Murray £25, pp400

So now we know - thanks to the prurient intercession of that woolly-pullied royal plaything Gyles Brandreth - what happened when the virginal Lilibet met her nautical Adonis at Dartmouth in 1939. He said: 'How do you do?' She replied: 'I'm quite well, thank you.' Oh, she also thought he looked 'achingly handsome'.

How does Brandreth know this? He doesn't, of course, as he at once admits: the tongue-tied dialogue he writes for the two teenagers is entirely invented. Indeed his entire biography of the pair can best be read as the product of a sweatily over-heated imagination. It's the kind of imagination a snob possesses, elated by a dizzy dream of high society and of his own exclusive access to it. This is a man who, while researching his novelettish tome, danced such obsequious attendance on the Queen that a helper at a hospice she was visiting took him for an equerry and asked if his employer needed the loo. Empowered to speak on behalf of the royal bladder, he officiously replied: 'She's fine, thank you.'

Brandreth even seems to have been sexually stimulated by proximity to the starchy monarch, and once ineptly flirted with her at a Buckingham Palace reception. On another occasion, he pressed his thigh against Princess Anne's under a dinner table. Alas, Anne's look of castrating contempt made him spend the rest of the evening with his knees girlishly clamped together.

Brandreth's wishful thinking about his intimacy with the Windsors licenses such friskiness. Hence his scoop about nocturnal arrangements: 'Although it is really none of your business (or mine), I am able to tell you that customarily the Queen and Prince Philip do share the same bed.' Note the way he blames us for his own intrusive curiosity, and accuses us of impropriety if we're silly enough to buy his fatuous book.

He's more circumspect about the honeymoon: 'What happened between the sheets on the night of the royal wedding I cannot tell you.' With a wince of regret, he adds: 'I was not there.' How could the newlyweds have forgotten to invite Brandreth, who would attend the opening of an envelope, let alone a royal defloration? The oversight must have been due to the fact that he wasn't born yet. He does, however, have a prize bit of carnal tattle about the next generation. He reports, on the authority of his fellow romancer Barbara Cartland, that Diana and Charles broke up because 'she wouldn't do oral sex, she just wouldn't'. The anecdote leaves me gulping with admiration for Camilla, apparently more adept at managing the princely sceptre and its pendulous orbs.

Otherwise, what does Brandreth have to tell us? His characterisation of Elizabeth reminds me of Gertrude Stein defining a rose: 'She is what she is and what she is is the Queen.' Asked by his friends what the Queen is like, Brandreth replies: 'Much as you'd expect, in fact.' He does have it on good authority that Elizabeth once told Philip to shut up, and - in the absence of anything more substantial - he tells the story half a dozen times. You can make up your own mind whether such apercus are worth £20.

With Philip, his task is to quash gossip not to relay it. Countess Mountbatten asserts: 'I am quite sure - quite sure, absolutely certain, he has never been unfaithful to the Queen.' Brandreth interviews a succession of supposed mistresses, who all insist that their friendships with Philip were innocently high-minded. He laments the need for discussion of such distasteful matters, then adds a smirking commercial aside: 'This is the chapter that will be serialised first.'

In return for the confidences of these hoity-toity ladies, he advertises their money-making enterprises. Potential day-trippers are supplied with the phone number of the Mountbatten estate, which can be visited for a fee. After his session with Sacha Abercorn, we are directed to the website which has turned her country house into a luxury hostelry. 'You can sleep,' Brandreth breathlessly promises, 'where the Queen slept.' He also acts as HM's estate agent, advising anyone who wants to honeymoon at Balmoral to check out the cottages for rent at the website there. Yet despite his own blend of snobbery and lubricity, he reproves a tourist who touches the nuptial bed at Broadlands and announces: 'The Queen is very keen on sex.' If you're interested, she is also partial to Oxford marmalade.

Flora Fraser's account of George III's progeny, Princesses: The Six Daughters of George III - six daughters and nine sons, all ejected from a proficient queen who typically spent 15 minutes in labour and then handed over the offspring to a milk-cow called Mrs Muttlebury - is a less flushed affair, and it openly tells the stories of infidelity, intrigue and rancorous discontent that Brandreth suppresses.

The Prince Regent's romps with Mrs Fitzherbert suggest that, for heirs to the throne, nothing much changes; the Regent's architectural follies anticipate Charles's spendthrift habits, which the Queen (according to Brandreth, for whom Charles is a villain) finds 'obscene'. Fraser's spurned Queen Caroline uncannily corresponds to Diana. When both fell from favour, the royal family ordered that their names be omitted from prayers for the monarchy. Caroline retaliated by attempting to gatecrash her husband's coronation, which makes me regret the fun we'll miss as a result of Diana's premature death.

George III was mad, and his daughters led opulently useless lives. The human family is a device for passing on property and emotional dysfunctions; the royal family hoards more than the average quantity of loot, and has more than its fair share of bad, recessive genes. Isn't it time that we released the Windsors from a duty which they find so onerous, and liberated ourselves from our absurd enthralment to them?

Brandreth, I suggest, might go into exile along with them. His performance as a self-appointed equerry-in-waiting that day at the hospice suggests that he'd be a dab hand with a loo brush and would keep their porcelain throne pristine.

Sign up for the Guardian Today

Our editors' picks for the day's top news and commentary delivered to your inbox each morning.